In recent years, a large capacity of information can be recorded on a disk having a small recording area with an increase in density of a disk apparatus. Thus, various kinds of technology to be used in a disk apparatus have been developed as recording media for mobile equipment, for example. A recording medium for mobile equipment may be required to be, apparently, small in size and weight, and have low power consumption and shock resistance in consideration of the usages.
FIG. 16 is a section view showing a first example of the clamp structure in a conventional disk apparatus. In FIG. 16, clamp member 3 has disk 1 between disk pressing portion 3g and disk receiving surface 2e of hub 2 through the axial force of screw thread 4. Disk 1 is integrally fixed to hub 2 by the frictional force of the contacts of the members. When a large shock is applied thereto beyond the frictional force, a phenomenon called disk shift occurs in which the position of disk 1 fixed to clamp member 3 and hub 2 is shifted largely. The disk shift is one of factors for causing a rotational runout of disk 1. Upon occurrence of the disk shift, the data track originally coaxial with the rotational center axis is largely decentered, which makes the precise following of a magnetic head (not shown) to the data track difficult.
A hard disk used as a recording medium for mobile equipment may require shock resistance, which guarantees a normal operation even after a shock beyond 1500 G is applied thereto at a state of not working. A construction for increasing the shock resistance is proposed as below. For example, in order to prevent the disk shift due to a strong shock, the frictional coefficient may be raised or the axial force of screw thread 4 may be increased in the disk apparatus as shown in FIG. 16 since the frictional force for fixing disk 1 needs to be increased. Therefore, the method for increasing the axial force has been conventionally adopted for reasons to be described below. That is, first of all, in addition to the demand for improvement of the precision in processing, the minuteness of the surface finish must be increased in order to prevent the adherence of burrs and/or contamination. Second, the disk shift due to the roughness of one surface of hub 2 and clamp member 3 or disk 1 (which increases the frictional coefficient) cannot be suppressed though the surface finish of hub 2, clamp member 3 and disk 1 is preferably minute since the technological developments tend to aim to reduce the amount of levitation more than the present amount of levitation of several tens of nano meter (nm) of the head above one side of disk 1. Furthermore, an increase in costs cannot be avoided even though the surface finish of the clamp area only excluding the data area can be rough technically. Therefore, the method for increasing the axial force has been adopted.
However, when shock resistance is increased by a large axial force, the repetitive runout of disk 1 is increased due to the clamping force caused when clamp member 3 is clamped. This means that the condition for increasing the shock resistance and the condition for resolving the repetitive runout of disk 1 due to the clamping force are mutually contradictory. Therefore, especially in the development of small disk apparatus, it is important that the improvement of shock resistance is compatible with the resolution of the repetitive runout especially in the development of a compact disk apparatus.
Furthermore, in order to minimize the repetitive runout of disk 1 due to an increase in clamping force, the center axis of disk pressing portion 3g must be coaxial with the center axes of disk receiving surface 2e of hub 2 and disk. The relationship will be described below with reference to the clamp structure in the conventional magnetic disk apparatus.
In FIG. 16, disk 1 is inserted into disk inserting portion 2a, which is the central projection of hub 2. One side of disk 1 is received by disk receiving surface 2e of hub 2. Clamp member 3 is mounted on the other side of disk 1 coaxially to disk 1 and is clamped between screw thread 4 and internal thread 2c at rotational axis 9 of hub 2. Since the diameter of screw head 4b of screw thread 4 is larger than the diameter of central hole 3a of clamp member 3 here, axial force occurs when screw thread 4 is clamped to internal thread 2c of rotational axis 9, which rotates on the inner circumferential surface of bearing sleeve 8. The axial force is transmitted from screw head face 4c to bottom face 3f near central hole 3a of clamp member 3, clamp member 3 coaxially and integrally fixes disk 1 to hub 2 with the other surface of disk 1 pressed in disk pressing portion 3g. Permanent magnet 7, which is a component of a motor for rotating hub 2, is fixed to the rim of hub 2. In order to use the magnetic force of the magnet of permanent magnet 7 effectively and suppress the leakage flux to the head, hub 2 is made from martensite steel.
In order to mount disk 1 and hub 2 coaxially, the central hole of disk 1 and the outer cylinder of hub inserting part 9a of rotational axis 9, which is slightly smaller than the diameter of the central hole of disk 1, are positioned and fitted to each other. Furthermore, in order to mount clamp member 3 and hub 2 coaxially, hub inserting portion 9a of rotational axis 9 and central hole 3a of clamp member 3, which has a slightly larger diameter than the diameter of hub inserting part 9a, are positioned and fitted to each other. The positioning may be achieved by placing hub inserting part 9a and clamp member 3 with a tab for engaging. The tab for engaging is provided such that clamp member 3 is not moved in the direction perpendicular to the central axis (that is, in the direction of the radius of disk 1). Clamp member 3 is bent for the thickness excluding the tab for engaging and is clamped by screw thread 4.
FIG. 17 is a section view showing a second example of the clamp structure in a conventional disk apparatus. The structure of the second example is different from that of the first example in that multiple screw threads 4 are provided on the circumference of clamp member 3 in order to suppress the runout in the rotational axis of disk 1 due to the inclination of clamp member 3.
While the above-described conventional disk apparatus has a construction including one disk only, a disk having recording areas on both sides or multiple disks may be used in order to achieve larger capacity. In a disk apparatus having multiple disks, the disks and spacers are alternately laminated between a hub and a clamp member, and the disks and spacers are pressed in the direction of the lamination by the outer area of the clamp member. Then, the disks and spacers are fixed to the hub. FIG. 18 is a section view showing a third example of the clamp structure in a conventional disk apparatus having multiple disks.
In FIG. 18, hub 121 can rotate around shaft 123 at bracket 122 through bearings 124 and 125. Rotor hub 126 is fixed to hub 121 and shaft 123. Stator 127 is attached to shaft 123, and rotor magnet 128 is attached to hub 121. The supply of current to the stator coil can rotate hub 121 along with rotor magnet 128. Disk 100 has direct contact with flange 129 of hub 121. Disk 110 and Disk 120 are fitted to hub 121 through spacer 130 before disk 100 and through spacer 131 before disk 100, respectively. Disk 120 is fitted to hub 121 by sandwiching spacer 131 with disk 110. Clamp 132 is fitted to rotor hub 126 and is fixed to hub 121 by screw thread 133. Clamp 132 presses disks 100, 110 and 120 and spacers 130 and 131 toward flange 129 and fixes them to hub 121. Flange 129 has projection 121a. This projection 121a is positioned on the outer side than the contact point between spacer 130 and disk 100, which is produced by the attachment of clamp 132 thereto, and on the surface of flange 129 facing toward disk 100. This projection 121a causes bending moment M1 in the opposite direction of the bending moment, which tries to deform the disk so that the disk can be maintained flat (see Japanese Patent Unexamined Publication No. H6-139675, for example).
In the conventional disk apparatus as described above, the recent increasing improvement of disk recording density has a problem that data cannot be read because a track shift is caused by displacement of the clamp member due to a change in temperature. In order to resolve the problem, a method has been proposed in which a clamp member and/or a disk spacer contain a material having a linear expansion coefficient substantially equal to that of a disk (see Japanese Patent Unexamined Publication No. H6-168536 and Japanese Patent Unexamined Publication No. 2002-133743, for example).
Furthermore, in order to build a disk apparatus in mobile equipment, more decrease in thickness of the apparatus itself has been demanded. In order to decrease the thickness, the thickness of a disk may be decreased, or a head for writing/reading may be placed on one side of the disk only. Alternatively, the recording capacity of one disk may be increased.
However, the construction of the first example has a problem that the direction and amount of a warp of the disk caused by the clamping of the disk cannot be controlled in a stable manner. Then, another problem occurs that the amount of levitation of the head is not stable, and, in a worst case, the head touches the disk.
The construction of the other proposed second example, that is, the clamping method in which disk 1 is fixed to hub 2 with multiple screw threads 4 has another problem that clamping screw threads 4 deforms a clamper or causes uneven disk pressing force. As shown in FIG. 19, another problem occurs that distortions and/or undulations, the number of which is equal to the number of screw threads, occur in the inner part of the disk. FIG. 19 is a diagram showing a deformation of a disk due to the clamping in a conventional disk apparatus.
The thickness of a disk itself tends to be decreased with decreases in thickness of the apparatus in recent years. As a result, the strength of a disk is decreased, and the distortion and/or undulation is/are increased in the direction of the circumference of the disk. Furthermore, the amount of warping is increased in the direction of the radius of the disk. The distortion and/or undulation of the inner part of the disk causes a change in levitation gap between head 20 and disk 1 instantly in the inner part of the disk while the disk apparatus is operating. For example, when three screws are used to clamp clamp member 3, a change occurs in head output in accordance with the number of screws every rotation of disk 1 as shown in FIG. 20. The percentage value of the value resulting from the division of the minimum value of the head output by the maximum value is called modulation. A small modulation deteriorates S/N of a read output signal in accordance with a change in output wave thereof, and an increase in time jitter of data reading pulses reduces the error rate, resulting in inaccurate data writing/reading. Furthermore, the positioning of head 20 to a target track and/or data writing/reading cannot be performed accurately. Still further, there is another problem that a decrease in amount of levitation causes a failure such as a contact between disk 1 and the head. FIG. 20 is a head signal output diagram in a conventional disk apparatus.
In the method in which the clamp member and disk spacer contain a material having a linear expansion coefficient substantially equal to that of a disk, the expansions and contradictions due to changes in temperature of a disk and the clamp member and disk spacer occur in a same manner, but, since the linear expansion coefficient of the hub containing a different material from that of a disk is larger than the linear expansion coefficient of the disk, distances Rh and Rs are changed where distance Rh is a distance from the rotational axis center of the motor to a contact point or plane between the hub and a disk, and distance Rs is a distance from the rotational axis center of the motor to a contact point or plane between the disk and the spacer ring, which are produced by the attachment of the clamp member. As a result, the amount and direction of warp of the disk change.
For example, when disk 1 is adjusted to be substantially flat without a warp as shown in FIG. 21A at a normal temperature (25° C.), distance Rh from the rotational axis center to the contact position between disk 1 and hub 2 to and distance Rs from the rotational axis center to the contact position between disk 1 and clamp member 3 side change relatively upon change in ambient temperature because of differences from the linear expansion coefficient (9.2 to 12×10−6) of hub 2 of martensite SUS and the linear expansion coefficient (6 to 7.2 to ×10−6 for glass and about 23.5×10−6 for aluminum) of clamp member 3, for example, of glass or aluminum. In other words, when the material of disk 1 is glass, distance Rh from the rotational axis center to the contact position between disk 1 and hub 2 moves to the outer part than distance Rs from the rotational axis center to the contact position between disk 1 and clamp member 3 since the growth rate of hub 2 is larger than the growth rate of clamp member 3 at a high temperature. Thus, disk 1 warps toward clamp member 3 in the direction of the outer circumference of disk 1 as shown in FIG. 21B. On the other hand, when the material of disk 1 is aluminum, distance Rs from the rotational axis center to the contact position between disk 1 and clamp member 3 to moves to the outer part than distance Rh from the rotational axis center to the contact position between disk 1 and hub 2 since the growth rate of clamp member 3 is larger than the growth rate of hub 2 at a high temperature similarly. Thus, disk 1 warps toward hub 2 in the direction of the outer circumference of disk 1 as shown in FIG. 21C. Conversely, in an environment at a low temperature, the opposite phenomenon of that at a high temperature occurs. Therefore, since the direction of the warp of the disk depends on changes in material and temperature, the levitation state of the head changes, resulting in a failure in writing/reading operations of the head disadvantageously.
On the other hand, in the construction of the disk apparatus including multiple disks, the hub is deformed by pressure applied by the clamp, and the disks are deformed in the direction of the diameter and warp. A smaller disk apparatus has a smaller spindle motor, a less stiff hub and a thinner disk, which produces larger deformation. In order to achieve larger capacity, not only the recording density of data on the disk but also the resolution of data recording must be increased, and the amount of levitation of the head therefore is significantly smaller. Furthermore, for a disk having a smaller diameter, the area for clamping the disk is shifted toward the inner radius. Therefore, the degree that the disk deforms tends to increase more.
The clamp structure in a conventional disk apparatus including multiple disks as shown in FIG. 18 has projection 121a at flange 129 such that projection 121a can be positioned on the outer side than the contact point between spacer 130 and disk 100, which is produced by the attachment of clamp 132. Because of projection 121a, the bending moment M1 is caused in the opposite direction of the bending moment trying to deform the disk so that the disk can be maintained flat. Furthermore, although the clamp structure in the construction considers to keep the deformation of the disks small or to keep the deformation of the disks caused by a change in environmental temperature small, the complete elimination of the deformation of a disk is significantly difficult with the disk securely maintained flat. Furthermore, the disks have a small deformation, a sufficient head output cannot be obtained in some direction of the deformation, which is another problem.
Furthermore, in the clamp structure in the conventional disk apparatus, the amount of deformation and/or warp of the disks caused by clamping are increased with a decrease in thickness of the disks. Therefore, the recording capacity may not be increased, and the stable levitation of the head cannot be achieved.